Machines of this type are commonly used for the production of tubes of cardboard or other sheet material on which to wind weblike material, such as paper, tissue paper, plastic film, aluminium sheets or the like. These tubes are usually circular in cross-section. Tubes thus produced can also have different shapes and cross sections, such as circular, square, rectangular, or other. Such tubes can be used not only as winding cores for the formation of rolls or logs of weblike material, but can also be designed for various different applications, as containers for foodstuff products, for soap powders, or for other applications. In sequel the present description reference will be made to the formation of circular tubes used as winding cores for weblike materials; however, the scope of protection of the present invention is not limited to this application, but it is understood as extending to all the sectors that regard formation of tubes from one or more wound strips of weblike material.
Winding of the weblike material can be obtained by winding one or more strips helically around the forming spindle, as represented and described with reference to the example of embodiment illustrated in what follows, or else they can be obtained by feeding longitudinally two or more strips that overlap one another until their sides are mated and wrap the forming spindle, as for example represented and described in WO-94/20281 (corresponding to U.S. Pat. No. 5,593,375).
Consequently, by the term “winding” it is to be Understood that the strips of weblike material can coat or wrap the forming spindle, it being possible for them to be fed to the latter either obliquely with respect to the axis of the spindle (helical winding), or else parallel to said axis (longitudinal winding). Consistently, a core-winder, machine for the production of tubes, or tube-forming machine should be understood as any machine in which strips of weblike material are wound around a mandrel to continuously form a tubular article of manufacture, such a winding cores, prismatic or cylindrical boxes and the like. Weblike material can be a cardboard strip, a plastic strip, or a strip of any other suitable material, depending upon the article of manufacture to be produced therewith. The web-like material strips can be adhered to one another by means of glue, adhesive, or any kind of bonding agent, by means of welding, such as ultrasound welding, or in any other suitable manner.
For the production of tubes of cardboard or other material via helical winding of one or more strips set staggered on top of one another a core-winder machine is used, typically comprising: a winding spindle, around which are wound helically the strip or strips to form the tube continuously, which is made to advance along the spindle; a device for supply and winding the strips around the spindle; at least one knife for cutting individual lengths of said tube being formed, which knife is provided with a reciprocating movement parallel to the spindle; a counter-knife within the tube being formed, provided with a movement of translation synchronized to the movement of translation of the knife.
A core-winder of the above type is described, for example, in U.S. Pat. No. 5,873,806. Other machines for helical winding of strips are described in U.S. Pat. Nos. 2,502,638, 2,623,445, 3,150,575, 3,220,320, 3,636,827, 3,942,418, 4,378,966, as well as in WO-A-2004101265 and WO-A-2004106017.
In these machines, the tube is formed continuously by winding of two or more strips of weblike material, for example paper or cardboard, staggered with respect to one another, around the winding spindle, which is mounted in cantilever fashion, either fixed or able to turn (preferably idle).
Irrespective of how the strips of weblike material are wound and adhered to one another, a continuous tube is usually produced, which must be cut into individual lengths that are designed for the final use, for example for winding of paper for the production of rolls. Cutting is executed with one or more disk-shaped knives that can be motor-driven or else idle and drawn in rotation by friction with the tube. The cutting edge of the knives can be smooth or serrated according to the configuration of the machine. The knives have an axis of rotation parallel to the axis of the spindle and hence of the tube being formed and are pressed against the outer cylindrical surface of the tube and advance together therewith parallel to the axis of the forming spindle. Usually, during rotation and advance of the tube, the cut is performed by the knife or knives according to a cutting plane orthogonal to the axis of the tube being formed. Once cutting has been completed, the knife is moved away from the axis of the tube and brought back into the position in which the next cut will start.
Usually, provided within the tube being formed is a counter-knife, with which the knife or knives set on the outside of the tube co-operate. Said counter-knife must follow the movement of the knife or knives during cutting and hence must advance in synchronism with the tube being formed up to completion of the cut and then come back into the position in which also the knife or knives move back to start the next cut. In some machines this movement is obtained by setting the counter-knife on a guide rod constituting a prolongation of the forming spindle and constraining the counter-knife temporarily to the knife as a result of the force of friction that is generated by pressing the knife on the material to be cut. In this way, the counter-knife advances together with the knife. When the latter is moved away from the tube, the counter-knife is recalled by a spring into its initial position.
The above constructively simple solution is not very reliable and entails in any case high pressure stresses between the knife and the counter-knife so that forces of friction will be generated sufficient to draw the counter-knife in the synchronous movement of advance with the knife. The aforesaid solution can moreover be critical in the case of particularly rigid tubes.
Furthermore, the return spring is frequently subject to failure due to fatigue, in so far as it must perform, in more modern machines, a travel of approximately 150 mm in every tenth of a second.
Other and more complex solutions envisage a positive system that advances the counter-knife during cutting and brings it back into its initial position during retraction of the knife.